The present invention relates to envelopes and more particularly to self-sealing envelopes and a mehtod for making the same.
A self-sealing envelope generally includes strips of latex material applied both to the flap of the envelope and to the body of the envelope. When the flap is closed, the flap strip bears against the body strip. Neither latex strip need be moistened in order to form a seal between the two.
The manufacture of self-sealing envelopes of the type described has not been free of problems. To reduce drying time for seam adhesive and latex and thus increase production rates, envelope manufacturers often use dielectric ovens or heating chambers. Dielectric heating chambers include two or more spaced electrodes connected to a high frequency oscillator. The oscillator output is generally in the range of 2,000 - 20,000 volts at frequencies ranging from 2 Megahertz to 90 Megahertz. The alternating electric fields established between the electrodes cause molecular agitation (i.e., heating) in the material to be dried. As a result, moisture is driven from the material in the form of vapor.
Where such dielectric heating chambers have been employed in the drying of self-sealing envelopes, a high electric field is necessary to dry the latex material. However, it has been found that the electric field intensity is greatest in those areas containing seam adhesive. The high electric field intensity and resulting concentration of generated heat can, and has, caused the envelope paper to become discolored, scorched and even burned along the seam. The same discoloration, scorching or burning has been found where an envelope window has been applied with a conventional adhesive.
Also, where latex material is applied over a previously-dried back seam, the dielectric drying of the latex material can still cause the latex to blister and discolor. It is believed that moisture in the latex adhesive penetrates the paper and enters the back seam adhesive.
The incidences of discoloration, etc. can be reduced by either reducing the operating voltage or the operating frequency of the high frequency oscillator for the dielectric heating chamber. However, since the drying effect of a dielectric heating chamber is equal to the product of the frequency and the square of the voltage, reducing either obviously reduces the efficiency of the chamber.
As an alternative a spark detector has been used in combination with the dielectric heating chamber. Whenever arcing is detected, the detector shuts down the high frequency oscillator. Intermittent operation of the high frequency oscillator naturally lowers the rate of production of the envelopes.